Dissociation between metabolic and contractile responses during intermittent isometric exercise in man

This study examines the temporal changes in high-energy phosphate and metabolic levels, and in force-generating capacity, during and after voluntary submaximal repetitive isometric exercise (RIE). Eight male subjects performed one-legged RIE with the knee extensors at 40% maximum voluntary contraction (MVC) target force (duty cycle: 6 s contraction, 4 s rest) in a 48 cm bore whole body 1.5 T superconducting magnet. Phosphocreatine (PCr), inorganic phosphate (P(i)), ATP and pH were measured every 9 s. Force-generating capacity was repeatedly measured using MVC force and electrically stimulated contractions (sequential train of impulses of 1-100 Hz). During RIE, MVC declined gradually by 56 +/- 5% (mean +/- S.E.M.). Electrically stimulated force also declined, with a disproportionally large drop in low-frequency force, seen as a decline from 0.76 +/- 0.02 to 0.33 +/- 0.02 in 20:50 Hz force ratio. The PCr decline during RIE was 65 +/- 9%, in most subjects seen as a rapid initial drop followed by less or no further decline to exhaustion. pH declined in parallel by 0.18 +/- 0.04 units, whilst ATP levels remained unchanged throughout the exercise. PCr, P(i) and pH recovered to near control values within 5 min of exhaustion. Force, however, was not fully restored after 30 min recovery. The results support the hypothesis that fatigue from submaximal RIE is unrelated to changes in P(i) and H+ levels. The decline in 20:50 Hz force ratio implies that fatigue may be associated with excitation-contraction coupling impairment. No sudden changes were observed in mechanical or metabolic factors at exhaustion. Exhaustion was probably not caused by lack of substrates for ATP resynthesis, since pH had decreased only marginally.